1. Field of the Invention
This invention relates to a method for etching and patterning a metal layer used in integrated circuit device fabrications. More particularly, this invention relates to a magnetically assisted reactive ion etch method for patterning a metal layer used in integrated circuit device fabrications, whereby a residue free patterned metal layer is formed without electrical charge up and explosion of the patterned metal layer.
2. Description of Related Art
A common feature in the production of integrated circuit devices is the use of metal deposition processes for the definition and fabrication of conductive pathways which connect various circuit elements.
Conventional metallization processes for the production of these conductive pathways have typically included a metal lift-off lithographic process whereby a photosensitive composition is first coated onto a semiconductor surface. The photosensitive composition is then masked, exposed to radiation and developed via an appropriate physical or chemical process to yield a final pattern of photoresist areas which define the locations on the semiconductor surface desired to be absent of metallization.
Subsequent to this photoresist patterning, a metal layer is deposited over both the exposed semiconductor surface areas and the patterned photoresist areas. In order to provide the final desired metallization pattern, the photoresist is then dissolved, removing with it the undesired metal layer which was deposited on the surface of the photoresist. Through this indirect metal lift-off process a metal pattern is provided on a semiconductor surface.
An unfortunate consequence of the indirect metal lift-off process for semiconductor surface metallizations is the production of substantial quantities of residual metal particulate upon dissolution of the photoresist upon which the metal has been deposited. These metal particles can easily redeposit onto adjoining semiconductor surfaces during the lift-off process, thus providing opportunities for voids or other semiconductor device fabrication defects.
As semiconductor device fabrication technology has matured and semiconductor device dimensions have decreased, alternatives to metal lift-off methods for semiconductor device metallization have evolved. A common alternative to the metal lift-off process is a metal etch process wherein the photolithographic definition of conductive metal pathways occurs by direct patterning of a metal layer which has already been deposited upon a semiconductor surface.
In this direct method for metal pattern formation, a photoresist is deposited, exposed and developed upon a previously deposited metal layer to yield a pattern of photoresist features which define the pattern of metal features to remain on the semiconductor surface. The exposed metal remaining between the developed photoresist features may then be etched away from the semiconductor surface through an appropriate chemical or physical etch process. Within this direct method for metal pattern formation, there are several choices of direct metal etch processes which, unlike the lift-off process, are not inherently susceptible to production of substantial quantities of particulate.
One of the more advanced methods used to directly etch metals from semiconductor surfaces without particulate formation is a Reactive Ion Etch (RIE) plasma process. Such plasmas are formed within a carefully designed reactor chamber by the exposure of appropriate gases to radio frequency energy. The gases must be chosen with specific consideration to the metal desired to be etched. In particular, the product obtained from the reaction between the gas plasma and the metal surface must be sufficiently volatile to be removed from the metal surface by the reactor system.
In the practice of reactive ion etching of a metal surface, the reactive gases are introduced into the reactor chamber at reduced pressure. The chamber is then energized through the introduction of Radio Frequency (RF) energy, which allows the reactive gases to transform into reactive species which in turn etch the exposed metal surfaces. Given an appropriate choice of metal etching parameters, metal etching will then occur without the formation of metal particles which can redeposit onto semiconductor surfaces.
Although the RIE process for plasma etching of metal layers can easily proceed in a contamination free fashion, it can nonetheless also be a comparatively slow method for removing exposed metal layers of substantial thicknesses. In order to provide more efficient removal of thick metal layers it has been found useful to increase the plasma density and enhance etch rates through introduction into the reaction chamber of a magnetic field which focuses and intensifies the RIE plasma species at the metal surface desired to be etched. Processes which accomplish this goal include Magnetically Enhanced Reactive Ion Etch (MERIE) processes and Electron Cyclotron Resonance (ECR) processes.
Although the provision of magnetic assistance provides enhanced etch rates of MERIE and ECR processes compared to RIE processes, it is nonetheless important that the magnetically assisted metal etching process not proceed at high rates at or near the final thicknesses of metal layer to be removed. Rapid metal removal rates at this endpoint may yield significant damage to surrounding unetched metal or other adjoining materials within the semiconductor device construction.
In order to provide adequate metal removal rate control near the endpoints of MERIE and ECR metal etch processes, it is common practice in the art to divide a MERIE or ECR metal etch process into a two step process. The first step is a main or primary metal etch whose etch parameters include radio frequency power and magnetic field strength of sufficient magnitude to quickly and effectively remove approximately all of the metal layer. This high power and high field strength primary metal etch is followed by a secondary metal over-etch process at substantially reduced power and field strength. The purpose of this secondary metal over-etch is to completely remove the metal layer and any other residues without damaging the surrounding semiconductor device structure.
Typically, the choice of radio frequency powers and magnetic field strengths for primary MERIE and ECR metal etches are approximately twice the equivalent values for the secondary metal over-etches. The magnitude of these changes in value is substantial. If these substantial changes are effected abruptly, damage may occur to the metal surface being etched. For example, it has been found that simultaneously and abruptly lowering both the radio frequency power and magnetic field strength for primary MERIE metal etches of semiconductor surfaces which contain multiple unconnected metallizations may cause those metallizations to experience unpredictable accumulations of electrical charge, causing them to explode and burst apart at unpredictable intervals.
It is thus the intent of this invention to provide a simple magnetically assisted multi-step metal etch process which provides residue free metallization patterns not susceptible to electrical charge up and explosion at the point of transition from a high radio frequency power and high magnetic field strength primary metal etch process step to a lower radio frequency power and lower magnetic field strength secondary metal over-etch process step. Although both MERIE and ECR processes are well known in the art of semiconductor metal etch processes, the art teaches neither the presence of a metal charge up and explosion problem associated with those processes nor a method to avoid that problem.